1. Field of the Invention
Embodiments disclosed herein relate generally to methods and apparatus for drilling and completing well bores. More specifically, embodiments disclosed herein relate to methods and apparatus for a drillable bridge plug.
2. Background Art
In drilling, completing, or reworking wells, it often becomes necessary to isolate particular zones within the well. In some applications, downhole tools, known as temporary or permanent bridge plugs, are inserted into the well to isolate zones. The purpose of the bridge plug is to isolate some portion of the well from another portion of the well. In some instances, perforations in the well in one section need to be isolated from perforations in another section of the well. In other situations, there may be a need to use a bridge plug to isolate the bottom of the well from the wellhead.
Drillable bridge plugs generally include a mandrel, a sealing element disposed around the mandrel, a plurality of backup rings disposed around the mandrel and adjacent the sealing element, an upper slip assembly and a lower slip assembly disposed around the mandrel, and an upper cone and a lower cone disposed around the mandrel adjacent the upper and lower slip assemblies, respectively. FIG. 1 shows a section view of a well 10 with a wellbore 12 having a bridge plug 15 disposed within a wellbore casing 20. The bridge plug 15 is typically attached to a setting tool and run into the hole on wire line or tubing (not shown), and then actuated with, for example, a hydraulic system. As illustrated in FIG. 1, the wellbore is sealed above and below the bridge plug so that oil migrating into the wellbore through perforations 23 will be directed to the surface of the well.
The drillable bridge plug may be set by wireline, coil tubing, or a conventional drill string. The plug may be placed in engagement with the lower end of a setting tool that includes a latch down mechanism and a ram. The plug is then lowered through the casing to the desired depth and oriented to the desired orientation. When setting the plug, a setting tool pulls upwardly on the mandrel, thereby pushing the upper and lower cones along the mandrel. This forces the upper and lower slip assemblies, backup rings, and the sealing element radially outward, thereby engaging the segmented slip assemblies with the inside wall of the casing. It has been found that once the plug is set, the slip assemblies may not be uniformly disposed around the inside wall of the casing. This non-uniform positions of the segmented slip assemblies results in uneven stress distribution on the segmented slip assemblies and the adjacent cones. An uneven stress distribution may limit the axial load capacities of the slip assemblies and casing, and reduce the collapse strength of the adjacent cones.
Further, due to the makeup or engagement of the backup rings adjacent the sealing element sealing element, the backup rings may provide an extrusion path for the sealing element. Extrusion of the sealing element causes loosening of the seal against the casing wall, and may therefore cause the downhole tool to leak.
Additionally, it has been found that downhole tools may leak at high pressures unless they include a means for increasing the seal energization, such as a pressure responsive self-energizing feature. Leakage occurs because even when a high setting force is used to set the downhole tool seals, once the setting force is removed, the ratchet system of the lock ring will retreat slightly before being arrested by the locking effect created when the sets of ratchet teeth mate firmly at the respective bases and apexes of each. This may cause a loosening of the seal. Downhole tools are also particularly prone to leak if fluid pressures on the packers are cycled from one direction to the other.
When it is desired to remove one or more of these bridge plugs from a wellbore, it is often simpler and less expensive to mill or drill them out rather than to implement a complex retrieving operation. In milling, a milling cutter is used to grind the tool, or at least the outer components thereof, out of the well bore. In drilling, a drill bit or mill is used to cut and grind up the components of the bridge plug to remove it from the wellbore. It has been found that when drilling up a bridge plug, lower components of the bridge plug may no longer engage the mandrel. Thus, as the drill rotates to drill up the plug, the lower components spin or rotate within the well. This spinning or rotation of the lower components during drilling of the plug increases the time required to drill up the plug.
Accordingly, there exists a need for a bridge plug that effectively seals a wellbore. Additionally, there exists a need for a bridge plug that may sustain a greater load capacity and increases the collapse strength of components of the bridge plug. Further, a bridge plug that is easier to drill up is also desired.